The invention relates to a thin-film transistor (TFT), wherein the TFT structure comprises a semiconductor material and electrodes connected to thereto in an electrically conductive manner.
Thin-film transistors are used mainly in flat-screen displays (LCD, OLED, E-paper, etc.). Especially for LCD-TV and OLED applications having a large surface, rapid electric circuits and/or high currents are needed. Copper (Cu) has clear advantages in this respect as compared to the currently most commonly employed Al electrodes and/or Al alloy electrodes.
The a-Si-Technology (amorphous silicon), which is the standard used to date, is difficult to combine with the Cu technology, since the Cu is exposed to temperatures of up to 400° C. during the deposition of the gate oxide and/or in the final CVD step for passivation of the stack of layers, and, for example, source/drain contacts undergo a diffusion reaction with Si in this process. Moreover, copper shows poor adhesion. For this reason, there is an ongoing search for suitable barrier/adhesive layers which, in addition, need to etch well together with the Cu.
Aside from the option to use a barrier layer, attempts are being made to use Cu alloys, in which alloy elements, such as Mg, Mo, W or Mn, are added, which precipitate at the layer boundary during layer growth or a downstream tempering step and thus intrinsically form a barrier and adhesive layer. However, no solutions that are suitable for production purposes have originated from these efforts yet. One main issue is that oxygen is always required for the precipitated element to show good adhesion and barrier function, since the a-Si layer itself has only pure Si at the surface. Attempts to overcome this issue are very laborious, since they require, for example, an initial in-situ oxidation of the surface of the a-Si transistor to be carried out. Moreover, this procedure is associated with the risk of obtaining an insulating boundary layer, which then prevents the desired ohmic contacting with the Cu electrode.
Another problem associated with the a-Si layers is that the current amplification varies and is too low. This is a problem mainly in the case of OLEDs.